The field of the present invention relates to heat exchangers and more particularly to heat exchanger systems in which a process fluid stream passes through two successive heat exchange steps where heat is exchanged between the fluid stream and two exchange fluids, and in which the operating conditions or physical properties such as temperature, corrosiveness, etc. of the fluid stream are aggressive with respect to ordinary materials of construction.
The successive exchange of heat between a fluid stream and two or more exchange fluids is practiced in many processes. The most common equipment configuration is a separate shell and tube heat exchanger for each exchange fluid with the fluid stream passing to successive exchangers through connecting pipes. For a typical shell and tube heat exchanger, the fluid stream may pass through either the shell side or the tube side of the exchanger, the best choice depending on the specific circumstances.
An example for successive heat exchange is found in an ammonia synthesis system such as that described in U.S. Pat. No. 4,744,966, where effluent gas (fluid stream) from a reactor is cooled in a first heat exchanger by exchange with reactor feed gas and then in a second exchanger by exchange with steam, the fluid stream flowing from the first exchanger to the second exchanger through connecting pipe.
When the conditions or properties such as temperature, corrosiveness, etc. of the process fluid are severe with respect to ordinary materials of construction, various designs are practiced to protect the shell of an exchanger from such an aggressive fluid.
Referring again to the specific example of U.S. Pat. No. 4,744,966, effluent gas from the first reactor leaves the reactor at a temperature between 480.degree. and 540.degree. C. and is conveyed into a first heat exchanger where it is cooled to a temperature between 390.degree. and 440.degree. C. by exchange with the feed gas to the first reactor. The effluent gas is then conveyed through a connecting pipe into a second heat exchanger where it is further cooled by exchange with steam. In this application the reactor effluent gas would normally be introduced into the tubes of the first exchanger by means of a connection such as that described in U.S. Pat. No. 4,554,135, to avoid subjecting the shell to the aggressive inlet gas. In the second exchanger, however, economics favor passing the gas through the shell side of the exchanger and the steam through the tube side.
It has been recognized by the inventors herein that at the lower part of the effluent gas temperature range, namely, below about 400.degree. C., a thin wall connecting pipe made of chromiummolybdenum alloy steel can be used. However, in the upper part of this temperature range, to avoid nitriding of the metal, a more expensive design would be required, using a much thicker pipe wall or an overlay inside the pipe made of a nitridingresistant material such as Inconel.RTM.. Regardless of the design, such high temperatures present difficulties in allowing for thermal expansion, differential expansion, maximum stresses and the protection of welded joints including a closing joint in welded construction.